High-performance alkaline water electrolyzers based on Ru-perturbed Cu nanoplatelets cathode

Alkaline electrolyzers generally produce hydrogen at current densities below 0.5 A/cm2. Here, we design a cost-effective and robust cathode, consisting of electrodeposited Ru nanoparticles (mass loading ~ 53 µg/cm2) on vertically oriented Cu nanoplatelet arrays grown on metallic meshes. Such cathode is coupled with an anode based on stacked stainless steel meshes, which outperform NiFe hydroxide catalysts. Our electrolyzers exhibit current densities as high as 1 A/cm2 at 1.69 V and 3.6 A/cm2 at 2 V, reaching the performances of proton-exchange membrane electrolyzers. Also, our electrolyzers stably operate in continuous (1 A/cm2 for over 300 h) and intermittent modes. A total production cost of US$2.09/kgH2 is foreseen for a 1 MW plant (30-year lifetime) based on the proposed electrode technology, meeting the worldwide targets (US$2–2.5/kgH2). Hence, the use of a small amount of Ru in cathodes (~0.04 gRu per kW) is a promising strategy to solve the dichotomy between the capital and operational expenditures of conventional alkaline electrolyzers for high-throughput operation, while facing the scarcity issues of Pt-group metals.


Supplementary investigations of the Ru@Cu-TM electrodes
where j, expressed in mA/cm 2 , is the current density used to reduce CuO into Cu shown in the plot of Supplementary Fig. 1b (i.e., 5 mA/cm 2 ); t, expressed in min, is the time for CuO reduction to Cu; F is the Faraday's constant (96485 C/mol) and n is the number of transferred electrons that reduced one CuO molecule to Cu (i.e., n = 2).
For the 1 cm 2 electrode shown in Supplementary Fig. 1b, the completion of CuO reduction is around 30 min, therefore the mass of Cu NPLs on TM is: mass of Cu NPLs = (Mw of Cu) × (Molar of Cu) = 0.0988 × t (mg/cm 2 ) = 0.0988 × 28 = 2.77 mg/cm 2 The mass of Cu NPLs calculated using the abovementioned equation is close to the ICP result, with an error of ca. 5%.  Fig. 2c) and recording EIS spectra (Supplementary Fig. 2d). The most performant electrode was obtained by potentiostatic electrodeposition of Ru NPs at -0.2 V vs. RHE. Indeed, such electrode exhibited a HER overpotential (referred to 0 V vs. RHE) of 84 mV (56 mV) at current density of -200 mA/cm 2 (-100 mA/cm 2 ), a Tafel slope of 31.3 mV/dec, indicating fast HER kinetics determined by Tafel reaction, and a small semicircle (corresponding to a Rct of 0.9 Ω) in EIS spectra (measured at -0.2 V vs. RHE), demonstrating the fastest charge transfer rate amongst the investigated electrodes.

Supplementary Figs. 3a,b
show the iR-corrected LSV curves measured for Ru@Cu-TM electrodes produced by varying the amount of Ru precursor in the electrodeposition bath (50 mL and 25 mL of 1 M NaOH, respectively). As can be seen, the higher concentration of Ru leads to higher performance of the resulted electrode. Interestingly, Supplementary Fig. 3b demonstrated that by decreasing the volume of electrolyte (from 50 mL to 25 mL), the dosage of Ru precursor could be thus significantly decreased (from 800 g to 500 g), with the performance of resulting electrode being well-maintained.
Besides, Supplementary Fig. 3c shows that the higher concentration of Ru ions could result in a faster deposition procedure for Ru NPs. i.e., if 800 g of K2RuCl6 is added to 50 mL electrolyte, corresponding to a 4 g Ru ion/mL electrolyte, Ru deposition could be done in around 8 h, while 500 g of K2RuCl6 in 25 mL electrolyte, corresponding to a 5 g Ru ion/mL electrolyte, could be finished in only 3 h. Impressively, a similar catalytic performance towards HER have been seen on the produced electrodes. The above-presented protocol optimization on maximizing the concentration of Ru ion by minimizing the volume of electrolyte is beneficial in term of both electrode production rate and cost.
The LSV analysis in Supplementary Fig. 3d demonstrates the reproducibility of Ru@Cu-TM electrodes using our optimized protocol: -0.2 V (vs. RHE) for Ru deposition, 500 g K2RuCl6 dosage, in 25 mL of 1 M NaOH electrolyte.  As shown in Supplementary Fig. 8a, Ti 2p peaks at 457.9 eV and 464.3 eV (cyan lines) were observed.
Since the binding energy of these peaks is slightly lower than typical Ti(IV), the Ti species in our Ru@Cu-TM are likely attributed to TiO2-x. 1 In Supplementary Fig. 8b, the the main peaks ascribed to metallic Ru (blue lines), while the presence of satellites are typically associated to RuO2 specie (magenta lines). Notably, C1s peaks at 284.8 eV and 288 eV could be attributed to C-C, C-H and C=O groups, respectively.  Fig. 13. Scaling-up of the fabrication of Ru@Cu-TM up to a 25 cm 2 (geometric) area. The synthesis procedure is similar to the 1cm 2 size case. Typically, 10 mmol copper (II) chloride dehydrate was dissolved into 200 mL Milli-Q water in a beaker, and 10 mL ammonia solution (25 %) was then added dropwise to get a blue solution. After then, an "L" shape TM with a rectangular area of 6 cm × 5 cm (a) was vertically placed into the beaker, which was then immersed in a preheated water bath (stabilized at 90 o C, as shown in b) for 3 h to obtain the CuO-TM electrode (c). The CuO-TM electrode was then immersed in 200 mL 1 M NaOH electrolyte and used as the working electrode in a three-electrode system (d), after which a negative current of -150 mA (-5 mA/cm 2 ) was applied until the electrode potential became stable, and bubbles (H2) evolved from the electrode surface. A 1 cm × 5 cm area of the obtained Cu-TM electrode was cut and left for backup use (e). The remaining 5 cm × 5 cm Cu-TM was put back to the electrolyte and used as the working electrode again, and a multistep galvanostatic protocol was performed, shortly after adding 12.5 mL K2RuCl6 solution (1 mg/mL in water) into the electrolyte (f), to obtain Ru@Cu-TM (g). The currents employed in multistep galvanostatic were extracted uniformly at different time points from the chronoamperometric plot shown in Supplementary Fig. 1d (5 cm 2 case, displayed as current density). The selection covers its entire time period of 5 h. Both Cu mesh (CM) and stainless-steel mesh (SSM) resulted in the formation of separated CuO aggregates, leading to an insufficient substrate coverage. These results indicate that the CuO NPL growth is strongly affected by the chemistry (affinity) of the substrate surface. Unlike Ti element being clearly observed in Ru@Cu catalyst grown on TM surface ( Supplementary  Figs. 6,7), the Ni element is hardly detected on Ru@Cu catalyst grown on NM, using SEM-EDS technique.
Supplementary Fig. 16. Investigation of HER kinetics on Ru@Cu-TM and Ru@Cu-NM cathodes. a, LSV curves with iR-correction. The Inset shows the corresponding Tafel plots. b, Equivalent electrical circuit used to fit EIS data measured for the cathode/electrolyte system. c, Bode plots and, d, corresponding Nyquist plots measured for Ru@Cu-NM at various overpotentials in 1 M NaOH. e, A fitting example extracted from (d). f, Bode plots and g, Nyquist plot measured for Ru@Cu-TM at various overpotentials in 1 M NaOH. h, Resistances (R2 and R3) of the equivalent electric circuit reported in (a) measured for Ru@Cu-NM and Ru@Cu-TM during operation in 1 M NaOH as a function of the potential vs. RHE.
As shown in Supplementary Fig. 16a, the Ru@Cu-NM cathode, showing similar structure of nanoplatelet arrays on the substrate surface (indicated in Supplementary Fig. 14d), performed worse than the Ru@Cu-TM ones, which could be due to the incorporation of Ti species into catalyst. To understand the difference of HER kinetics of Ru@Cu based on TM and NM, EIS was performed under various overpotentials. As is known, the HER proceeds through either Volmer-Heyrovsky or Volmer-Tafel mechanism or mix. The Volmer step (H2O + e -⇄ Hads + OH -, in which Hads refer to adsorbed hydrogen) and Heyrovsky step (H2O + Hads + e -⇄ H2 + OH -) involve one electron charge transfer, and therefore could be recorded by EIS technique, while no electron transfer occurs during Tafel step (2Hads ⇄ H2). According to previous reports, the Volmer step (hydrogen adsorption at the catalyst surface) is associated to the lowfrequency region. The medium/high (middle)-frequency region is related to the Heyrovsky step (charge transfer reaction at electrolyte-catalyst interface), and the high-frequency region is correlated to the electron transfer within catalyst inner-layer. 2,3 By referring to the electrical equivalent circuit for the cathode/electrolyte interface reported in Supplementary Fig. 16b, the resistance of the Volmer step  (R2), the resistance of the Heyrovsky step (R3) and the resistance of the electron transfer within the catalyst layer (R1) can be calculated as function of the HER overpotential. Supplementary Fig. 16c,d show the Bode plots and the corresponding Nyquist plots, respectively, measured for Ru@Cu-NM at various potentials in 1 M NaOH. The data are compared to those measured for Ru@Cu-TM ( Supplementary Fig. 16f,g). As depicted in Supplementary Fig. 16h, compared to Ru@Cu-TM, Ru@Cu-NM displayed higher resistance for both the Volmer step (R2) and the Heyrovsky step (R3) within the investigated potential range (from 25 mV to -100 mV vs. RHE). This means that the TiO2 species incorporated into the catalyst (see Supplementary Figs. 6-8) can act as efficient water dissociation centers, in accordance with previous reports. 4,5 Regarding the Ru@Cu grown on NM, NiO is marginally incorporated into the catalysts (Supplementary Fig. 15) and does not actively participate in the Volmer step, as evidenced by the EIS analysis.

Experimental data on hydrogen adsorption free energy
Experimental adsorption energies and DFT overbinding. The heats of adsorption of H2 (i.e. the adsorption enthalpies) were taken from ref. 6 (Cu) and 7 (Ru). To compare it with the DFT-calculated values, the zero-point energy correction (0.04 eV, as determined in ref. 8 ) was subtracted, resulting in the following values for the adsorption energies: -0.17 eV (Cu) and -0.48 eV (Ru). The εPBE values (i.e., the DFT overbinding, see Methods section of the main text) were obtained as the difference between the abovementioned experimental values and the calculated ones (Supplementary Table 2). Note that for the hydrogen adsorption on Cu, a fairly wide range of experimental values was reported, depending on the structural details of the Cu film. The authors adopt as final values for the adsorption energies the range 40-50 kJ/mol (note that experimental values refer to the H2 molecule, while the values discussed in our work are all normalized to a single H atom, i.e., the experimental values have to be halved to be compared to our simulations). We adopt the lower bound of that range because the experimental values obtained by other authors, discussed in ref. 6 , are almost all lower than 40 kJ/mol.

Entropy of adsorption.
While in the landmark paper by Nørskov et al. 8 the entropy of the adsorbed hydrogen was neglected, the analysis of experimental data in ref. 6 shows that that entropy reaches the considerable value of 60 J mol -1 K -1 , corresponding to an entropic contribution (TΔS) to the adsorption free energy at T = 300 K of 0.11 eV (normalized to ½ H2, see previous paragraph) rather than 0.21 eV of ref. 8 . Note that, from ref. 6 , we took the entropy value corresponding to the lowest hydrogen coverage, as the hydrogen coverage is low in our simulations as well (only one H atom is adsorbed in a large unit cell).

Investigations of anodes
To have a suitable anode benchmark for the evaluation of the AELs hereafter, we first developed an efficient OER catalyst taking inspiration from previous reports. 9,10 More in detail, the immersion of precleaned NF in a solution of Fe(NO3)3 and Ni(NO3)2 at 80 o C was found to be a simple and effective method to grow OER-active Ni-Fe hydroxides with low crystallinity onto NF (NiFe@NF) (Supplementary Figs. 17a,b), capable to operate at current densities higher than 100 mA/cm 2 (see details of fabrication in Experimental Procedures). Various concentrations of Ni and Fe precursors, reaction temperature (50 o C and 80 o C) and treatment time (0.5 h, 1 h, 3 h and 6 h) were screened to evaluate their impact on the OER activity of the resulting electrodes. The latter are named NixFey_Xh@Y@NF, in which x and y indicate the Ni and Fe precursor concentrations, respectively, (expressed in mM), X refers to the treatment time (expressed in h), and Y indicates the reaction temperature (expressed in °C). As shown from the analysis of the OER overpotentials (referred to 1.23 V vs. RHE) at a fixed current density (Supplementary Fig. 17c), the Ni50Fe50_3h@80@NF was found to be the most performant anode. The SEM-EDS analysis demonstrates that Ni, Fe, and O elements were uniformly dispersed throughout the catalyst ( Supplementary Fig. 18), with the atomic ratio of Ni/Fe being 2.17/1, nearly consistent with the results from ICP analysis (1.95/1). The XRD analysis, in turn, was characterized by a broad and weak diffraction peak at ~11.5° indicating the amorphous nature of the NiFe hydroxide ( Supplementary Fig. 19).
Electrochemical measurements indicated that our optimized anode (hereafter simply named NiFe@NF) exhibited a current density of 200 mA/cm 2 at an overpotential as low as 267 mV, and worked at such current density for 200 h with only ~25 mV overpotential increase (Supplementary Fig. 17d,e).
Notably, NiFe@NF did not show any significant morphology change after the CP test, further proving its electrochemical stability under high-current density operating conditions (Supplementary Fig. 17f).
In addition, the inset SEM in Supplementary Fig. 17f indicated that the thickness of the NiFe layer on NF was ~2 µm. After the CP test, the XRD pattern of the NiFe powder collected from the NF surface ( Supplementary Fig. 19) was analogous to that of the starting catalyst. Meanwhile, the EDS analysis revealed a partial Fe loss (~11 %), with the atomic ratio of Ni/Fe slightly increased to 2.42/1 (Supplementary Fig. 20), in agreement with the data obtained by the ICP analysis (2.27/1). A similar Fe leaching from the catalyst during OER operation was also reported in previous works, investigating the stability of NiFe-(oxy)hydroxide anodes in industrially relevant environments. 11 Notably, the NiFe@NF changed its color from the initial brownish to black after performing the OER (Supplementary Fig. 21a). We associate this effect to the oxidation of NiFe hydroxide to NiFe (oxy)hydroxide. 12 The performance of our NiFe@NF anode was then compared with those of NF, single or stacked SSM, as well as with the literature and/or commercially viable benchmarks, i.e., hierarchically structured Ni-Fe on NF 13 and NiFe2O4 particles on a 316L sintered stainless steel fiber felt (NiFe2O4@SSFF) 14 . In addition, NiFe grown on NM by replacing NF was also evaluated to understand the effect from substrate structure. As shown in Supplementary Figs. 17g,h, our optimized NiFe@NF had the highest OER activity amongst the investigated anodes, and could outperform most of previously reported Ni-Febased OER electrocatalysts, especially at current densities higher than 100 mA/cm 2 ( Supplementary  Fig. 17i, Supplementary Table 4). Lastly, we demonstrated that the upscaling procedure does not affect the geometric activity of the NiFe@NF anode ( Supplementary Fig. 21b), confirming its suitability as benchmark anode for AELs. When compared to cathode/anode combinations reported previously for water electrolysis, the Ru@Cu-TM / NiFe@NF pair is expected to reach current densities of 10 and 200 mA/cm 2 at cell voltages of 1.45 and 1.58 V, respectively, in 1 M NaOH and at ~ 27 °C. Even if not of practical interest for AELs (whose characterization is reported hereafter), the cell voltage at 10 mA/cm 2 current density is one of the smallest ones among those reported in literature for cathode/anode pairs potentially considered for the design of high-efficiency AELs (Supplementary Fig. 22 and Supplementary Table 5    Although carbon rod is a recommended CE for the HER, it is not stable due to its low oxidation potential being 0.207 V vs. RHE. Consequently, its oxidation becomes severe and causes the release of carbon ash under high working current (Supplementary Fig. 23a). Hence, we are keen to use winded Pt wire as CE, while the absence of XPS peaks at the positions, marked in the Supplementary Fig. 23b, in Pt 4d, Pt 4f and Pt 5p spectra acquired on the as-produced Ru@Cu-TM excluded the presence of Pt.
Besides, we later replaced the winded Pt wire by Rh deposited Ni mesh (Rh@NM) with a much bigger geometric size of 24 cm 2 (see the inset photo in Supplementary Fig. 23c). On the one hand, the Rh provides a more robust stability than Pt in terms of corrosion. On the other hand, the much larger CE surface (compared to WE) would avoid/minimize the corrosion of CE and could therefore become appropriate for HER study in alkaline. 15 As can be seen, the produced electrodes using winded Pt wire and Rh@NM (24 cm 2 ) demonstrate similar activity towards alkaline HER, supporting that the use of Pt wire is not the reason for the good performance of our produced electrode.
In short, we acknowledge that the use of Pt is not a perfect choice, and the selection of an appropriate CE for HER research under high operating current conditions remains an unsolved problem. 16  As shown in Supplementary Fig. 25, the addition of extra GDLs (e.g., carbon paper -CPR-GDL at the cathode side and platinized Ti fiber felt -Pt@TFF-GDL at the anode sides) increases the AEL performance (1 A/cm 2 at 1.82 V; 1.86 A/cm 2 at 2 V) compared with GDL-free AEL (e.g., 1 A/cm 2 at 1.84 V; 1.63 A/cm 2 at 2 V). Notably, CPR GDL was not evaluated at the anode side because of the corrosion of carbonaceous materials at potential higher than 1.23 V vs. RHE necessary for the OER (equilibrium potential of carbon = 0.207 V vs. RHE). 17 Meanwhile, Pt@TFF was initially considered as performance-ideal GDL candidate because the Pt coating can prevent the oxidation of Ti during OER conditions, ensuring optimal electrocatalysts/GDL/bipolar plate electrical contact for long-term AEL performance. Indeed, CPR GDL at the cathode side and Pt@TFF at the anode side lead to a slight increase of the AEL performance (e.g., 1 A/cm 2 at 1.82 V; 1.86 A/cm 2 at 2 V) compared with only CPR GDL at the cathode side (e.g., 1 A/cm 2 at 1.83 V; 1.74 A/cm 2 at 2 V). Noteworthy, Pt@TFF at the cathode side deteriorates the GDL-free AEL performance. Similar effects have been previously observed for Ti fiber felts GDL in PEM AELs. 18 Despite the further addition of Pt@TFF GDL at anode could slightly increase its performance, its high cost, due to the presence of Pt (as well as Ti), motivated us to remove it for the subsequent tests. As can be observed in Supplementary Fig. 26, the AEL performances were approximately preserved with upscaling the electrode active area from 1 cm 2 to 5 cm 2 , obtaining 0.5 A/cm 2 at 1.68 V, 1 A/cm 2 at 1.85 V. Along with our AELs, we assembled also two AEM-ELs using commercially viable AEMs, i.e., Sustainion X37-50 grade 60 and Fumasep FAA-3-PK. However, in the investigated electrolyte (1 M KOH) at which AEM commonly operates, 14 the assembled AEM-ELs performed significantly worse than our AELs based on Zirfon diaphragm (Supplementary Fig. 29). Noteworthy, for the case of Sustainion X37-50 grade 60 AEM, the assembly process typically resulted in damaging the AEMs, causing the short circuit of cells. To be able to evaluate the AEM-ELs, we specially reduced the electrode stack compression against the AEM, and this may have caused insufficient electrical contact between electrodes and bipolar plates.
Moving to the HFR recorded on different separators, the thinner Zirfon Perl UTP220 indeed demonstrated a much lower resistance (average HFR: 0.067 Ω cm 2 ), compared to either its thick counterpart of Zirfon Perl UTP500+ (average HFR: 0.130 Ω cm 2 , similar as previously reported value 19 ) or Fumasep FAA-3-PK (HFR: 0.300 Ω cm 2 ). Notably, the Sustainion X37-35 AEM displayed much higher resistance (average HFR: 0.330 Ω cm 2 ) than that of Zirfon diaphragm. We note that special precaution was taken at handling Sustainion X37-35 membranes, since the sharp features of NiFe-NF often led to the break of this AEM. Therefore, we have increased the thickness of the spacers, which however could have led to an insufficient compression of the cell stack, causing large resistances. A commercially available dual-feed AEM-EL (Dioxide Materials, NiFeCo || NiFe2O4, Sustainion ® AEM, 5 cm 2 ) was also investigated to benchmark our AELs. The performance of the commercial AEM-ELs in 1 M KOH (1 A/cm 2 at 1.9 V) has been reported in previous works, 14 and is inferior to those of our AEL (1 A/cm 2 at 1.69 V). In our operating conditions, our AEL outperformed the AEM-EL (Supplementary Fig. 30)  As shown in Supplementary Fig. 33a (Supplementary Fig.  33b). After the AST, the SSMs of our AEL anode did not show any relevant morphological change compared to fresh SSMs. Additional morphological and chemical analyses of anode SSMs after longterm (1000 h) AEL operation at 1 A/cm 2 is reported later in Supplementary Fig. 41. The EDS maps acquired on the Ru@Cu-TM catalyst collected from the electrode substrate after AST procedure indicated a Ru content (2.97 atomic percentage -at%-, based on total amount of Ru and Cu) (Supplementary Fig. 34), which is still comparable to the fresh sample (2.25 at%) ( Supplementary  Fig. 6). As previously discussed, the formation of soluble CuOH species in harsh alkaline media and their subsequent redeposition under negative potential (-0.2V vs. RHE in our case), causing the transformation of Cu NPLs into Cu NWs, as well as the dissolution/redeposition of Ti species, may explain the slight change of EDS-detected Ru at% of the cathode surface. As shown in Supplementary Fig. 35, the TEM images of the catalyst powder collected from Ru@Cu-TM cathode after the 24 h-AST indicated the presence of two different morphologies. The morphology shown in Supplementary Fig. 35a is mainly made of NPLs, together with the occasional presence of NW (previously recognized as NWs from SEM imaging), as observed on the surface of the cathode facing the CPR GDL (see SEM analysis of Supplementary Fig. 33b). The morphology of NPLs is similar to that observed for NPLs in the fresh cathode (see Supplementary Fig. 5). Supplementary  Fig. 35b, instead, shows a TEM image evidencing the main presence of NWs, as observed on the surface of the cathode facing the Zirfon Perl UTP220 diaphragm (see Supplementary Fig. 33a). As shown in Supplementary Fig. 36a, the HAADF-STEM and EDS-mapping analysis of Ru@Cu-TM catalyst after the AST indicated that Cu, Ru, Ti, O and K are present along the nanowire structure, while Ru NPs is still present atop the NW, without aggregating. As explained in Supplementary Fig. 34, the initial Cu NPLs could transform into Cu NWs due to formation of soluble CuOH species in alkaline media and their subsequent redeposition under negative working potentials. Meanwhile, the etching of Ti substrate and redeposition of dissolved Ti ions onto Cu can also take place, leading to the growth of TiO2 on the Cu NWs. Indeed, from the EDS data, the Ti amount increase after the AST compared to the fresh sample. The significant amount of K along with the NW structure, is attributed to the K contamination resulting from the AEL electrolyte (30 wt% KOH). Supplementary Fig. 36b shows the HRTEM images of a Ru NP, together with the corresponding STEM-EDS spectrum, evidencing that Ru NPs remain incorporated into the catalysts during AEL operation, without obvious morphological changes (Supplementary Fig. 7). Bundle-like Cu NWs were observed on Ru@Cu-TM after 608 h quasi-continuous stability operation (Supplementary Fig. 38a,b). In the manuscript, we ascribed such morphological transformation to the in-situ dynamical nanostructuring of the Cu NPLs during operation at large current densities in harsh alkaline media. Indeed, the presence of CuOH species on Cu electrode has been evidenced up to -1.3 V vs. SCE (ca. -0.3 V vs. RHE) in 0.5 M NaOH, while no formation of oxidized Cu was found at -1.4 V vs. SCE. 20 Also, Cu adatoms can undergo oxidation to form CuOH even under more negative potentials in 1 M NaOH. 21 Thus, the change of the Ru@Cu-TM morphology can be associated to a complex kinetic competition between the oxidation of metallic Cu under alkaline conditions and the reduction of Cu species under the application of a cathodic potential.

Cu-KA
Regarding the anode, the SSMs composing our anode (i.e., 5-stacked SSMs) exhibited some surface cracks after 608 h of quasi-continuous operation (Supplementary Fig. 38c,d). Such cracks can be ascribed to the oxidation/corrosion phenomena leading to the progressive formation of active FeO, Fe2O3 and NiO, which act as the active species for the OER. 22 The EDS maps shown in Supplementary Fig. 39 acquired on the Ru@Cu-TM surface still evidenced the presence of Ru and Cu, together with increased Ti amount compared to fresh sample (Supplementary Fig 6), which could be ascribed to the dissolution (i.e., TM etching) and redisposition of Ti species on the surface of Cu NWs in harsh alkaline media (also see in Supplementary Fig. 40).
Supplementary Fig. 40. Proposed morphology evolution of Ru@Cu-TM cathode during AEL operation.
As shown in Supplementary Fig. 40 and discussed in the manuscript, the Cu NPLs gradually change to Cu NWs due to formation of soluble CuOH species in alkaline media and their subsequent redeposition under negative working potentials, during which dissolved Ti ions (e.g., HTiO3 -) originated by the etching of TM can redeposit onto Cu NPL/NW surface. Supplementary Fig. 41. XRD patterens of (a) Ru@Cu-TM cathode side facing Zirfon Perl UTP220 diaphragm and carbon paper (CPR), respectively, after 24 h-AST. (b) XRD patterns of fresh SSM, and SSMs after AST and 608 h of quasi-continuous stability test of the corresponding AELs.. Supplementary Fig. 41a, the XRD pattern of Ru@Cu-TM cathode facing the CPR GDL showed intense peak for the phases of Cu and and Cu2O after 24 h-AST, which is in accordance with the SEM results (see Supplementary Fig. 33a,b), where the cathode surface facing CPR GDL retained the catalyst deposits after AEL disassembling. Contrary, detachment of catalyst layer from cathode surface and transfer to adhesive Zirfon Perl UTP220 diaphragm was observed for the cathode surface facing the diaphragm, resulting in less intense peaks ascribed to Cu species. The catalysts detachment from the cathode surface to the diaphragm was determined by the pressure applied to the (quasi) zerogap AEL stack, as well as by the adhesivity of the Zirfon Perl UTP220, and does not represent an issue for the AEL operation.

As shown in
The peak at 2θ of ~26.5° on cathode surface facing CPR GDL was ascribed to the carbonaceous residual of CPR. Some additional peaks could be assigned to K2CO3, which is formed by the reaction between residual electrolyte of KOH and CO2 from air.
Lastly, no extra peaks were observed on the SSM anode after the stability tests of the corresponding AELs (24 h-AST and 608 h of quasi-continuous operation at 1 A/cm 2 ), compared to the fresh one (Supplementary Fig. 41b). 20  The Pt/C cathode was prepared through spray coating of inks of 20 wt% Pt/C in water:isopropanol (75:25), which were produced with a Pt/C concentration of 1 mg/mL and adding an amount ofa Nafion dispersion (10 wt%) corresponding to a 25 wt% Nafion content relatively to the solid content (i.e., Pt/C + Nafion). This cathode recipe was optimized in ref. 24 . The inks were sonicated in an Ultrasonic Bath USC-THD (WVR) for 1 h to get homogeneous dispersion. The so-produced inks were hand sprayed on CPR mounted on a hot plate pre-heated at 140°C, and the catalyst mass loading, i.e., mPt, was set to 150 μg/cm 2 by adjusting the amount of the sprayed ink.
As shown in Supplementary Fig. 42a, the benchmark Pt/C-CPR || 5-stacked SSMs AEL, based on the same configuration as Ru@Cu-TM || 5-stacked SSMs AEL, demonstrated a stable performance for 1000 h operation at 1 A/cm 2 . However, we pointed out that after stability test, the composition of SSM surface changed compared to its initial state, thus progressively improving its catalytic activity towards the OER. In particular, Ni was progressively exposed at the SSM surface, suggesting the leaching of Fe and Cr (Supplementary Figs. 42b-d). Thus, SSM represent a promising commercially available and cost-effective anode for high-performance AELs operating with 30 wt% KOH electrolyte at 80 o C, as also indicated by recent studies. 22,23 Supplementary Fig. 43. Polarization curves measured for capillarity-fed Ru@Cu-TM || NiFe@NF AELs at room temperature and at 80 °C, and the AELs including nonwoven wipers as additional spacers. Test conditions: CPR GDL at Cat. and none at Ano.; electrolyte: 30 wt% KOH. The room temperature electrode-level characterization of the cathodes (see Supplementary Figs. 2a,f in the main text) intrinsically proves their potential for E-TAC systems. Indeed, the HER process is the same as in alkaline electrolysis, except that it occurs at ambient temperature instead of high temperature (e.g., 80 °C as for the case of the proposed AELs). Thus, the designed cathodes can represent advantageous alternatives to traditional cathodes, e.g., Raney-Ni 25 or NiMo electrodes and other HER electrocatalysts, e.g., platinized Ni-coated SSM 26 . Based on this rationale, a preliminary study on capillarity-fed AELs was also carried out using Ru@Cu-TM || NiFe@NF AEL configuration (Supplementary Fig. 43). At room temperature, the proposed capillarity-fed AELs reached ~0.20 A/cm 2 at 2.0 V. By increasing the temperature up to 80 °C, the cell reached 0.56 A/cm 2 at 2.0 V. Lastly, two pieces of nonwoven wipers were added as extra spacers, providing additional pathways for the capillarity-induced transport of the electrolyte beyond the one given by Zirfon Perl UTP 220 diaphragm. Thus, at 80 °C, the as-modified capillarity-fed AELs reached a current density as high as 0.82 A/cm 2 at 2.0 V. 3-layer slabs were adopted that measured 12.8 × 13.3 Å (Cu) and 13.6 × 14.1 Å (Ru). 3×3×1 gamma-centered k-points grids were adopted. All other settings and approximations are as described in the Methods section of the main text. Carbon paper (CP) was adopted as GDL at cathode side in our AEL assembly, while no GDL was used at anode side.

Supplementary
The cell voltage values with the symbol (~) are extracted from their corresponding polarization plots.
The operating pressure of ELs is not always mentioned.

Estimation of operating cost for H2 production in our AELs
In this section, the operating cost for the H2 production in our AELs were explicitly calculated by using the experimental data acquired during the stability test of our Ru@Cu-TM || 5-stacked SSMs AEL using Zirfon Perl UTP220 diaphragm. In particular, an average voltage of 1.71 V was measured for the AEL operating at 1 A/cm 2 for 500 h.

Techno-economic analysis of H2 production at MW-scale AEL plant-level
The following section, reported as a bullet list for the sake of clarity, aims at guiding the reader in the navigation and understanding of the attached Excel file, in which all the calculations related to the TEA of our AEL technology are gathered.
Sheet "Unitary cost of DEP components": evaluation of the unitary cost of the cell components, including: 1. determination of the unitary prices of raw materials composing each component of the DEP (Supplementary Table 7). In particular, for cathodes and anodes, determination of the mass loading (by ICP-OES) of the elements found in the catalytic layer; 2. from the data collected in the previous point, calculation of the raw materials cost associated to each synthesized/fabricated cathode/anode, by multiplying the unitary price of the metal by its mass loading; 3. evaluation of the fabrication cost of cathodes and anodes according to the synthetic/fabrication procedure (refer to the Experimental, section 4 of the main text) and the related parameters and costs (Supplementary Table 8). In the following, the equations used for calculating each single contribution to the overall cost of manufacture are reported:  is the power fed to the AEL plant to carry out the water splitting reactions; OPEXElectricity is the AEL plant OPEX contribution of the electricity fed to the AEL plant to carry out the water splitting process. CElectricty is the cost of the electricity According to reports on currently operative AEL plants, 28 the energy consumption for the water splitting process accounts only for the 50% of the overall energy fed to the whole AEL plant, with BoP auxiliaries (e.g., gas and liquid circulation, and gas compression) requiring a similar energy fed. Doubling the OPEXElectricity allows to obtain an overall OPEX more adherent to a real case scenario. where mH2O consumed (per year) is the mass of the water consumed per year, the average water consumption per kg of produced H2, m(Average per kg of H2) is set at 10 L kg -1 and the average cost of water (CH2O) is set at 0.0014 $ L -1 (Supplementary Table 10); 4. labor, maintenance and other ancillary expenses are instead calculated as standardized percentages of the initial CAPEX of the whole system. Specifically, the annual OPEX associated to labor, maintenance and ancillary activities are estimated to equal the 0.3, 2.5 and 1% of the initial CAPEX, respectively.
Sheet "Annual H2 productivity at 1 MW": Refer to the dedicated paragraph in the Experimental, section 4 of the main text.
Sheet "H2 production cost": Refer to the dedicated paragraph in the Experimental, section 4 of the main text.
Sheet "Ru availability": reports a rough assessment of the sustainability of the technology in terms of raw materials worldwide availability versus the deployed power envisaged according to IRENA's Energy Scenarios: 28 1. Starting from the determined Ru mass loading and considering the electrode area, the number of cells per stack and the number of stacks per electrolyzer (all these parameters have been calculated/assumed in the sheets "CAPEX -1 MW ideal AEL" and "OPEX -1 MW ideal AEL"), calculation of the Ru content per deployed power according to the formula: where LoadingRu is the Ru mass loading assessed by ICP-OES on our electrodes (ca. 53 g cm -2 ) while all the other parameters retain the above-described meaning.
2. Considering the cost and the worldwide production/reserves of Ru ("Additional data") and taking into account the total electrolyzers' power to be deployed globally by 2030 and/or 2050 according to IRENA's Planned/Transforming Energy Scenarios, 28 calculation of the mass of Ru demand related to our technology. 3. As with Ru, calculation of the demand/supply ratio of Pt when considering PEMELs as the technology of reference for the deployment of the electrolysis power envisage by IRENA's Planned/Transforming Energy Scenarios Ni-based commercial AEL system: The above reported description applies also to the Excel file dedicated to the commercial Ni-based system (1 Cell Alkaline Electrolysis Stack with 12 cm 2 active size -Fuel Cell Corp.). 29 As the electrochemical performance of the system has been recorded up to a current density of 500 mA/cm 2 , the cell voltage at 1 A/cm 2 has been linearly extrapolated from the trend reported in Supplementary Fig. 44, green line.

Cathodes fabrication Parameters
CuO chemical deposition Electricity cost 0.02 $/kWh a The cost of the heating of the chemical baths was calculated considering the energy required to heat the baths from room temperature up to the desired temperature, as ΔQ = mChemical bath × Cp × ΔT, where ΔQ is the heat energy, ΔT is the temperature change, mChemical bath is the mass of the chemical bath solution and Cp is the heat capacity of the solution (approximated to that of pure water, CpH2O). A perfect thermal insulation of the bath itself was postulated in the calculation of ΔQ. CpH2O = 4.187 J g -1 °C -1 , while the water density (dH2O = 1 mg mL -1 ) was used to calculated mChemical bath from the bath volume.  Fig. 11 To control the growth of CuO NPL on CM and SSM substrates as for the case of TM and NM, a layer of Ti was preliminary sputtered on CM and SSM. Supplementary Fig. 45 shows that CuO NPLs can grow vertically on Ti-coated NM and SSM. In addition, the thickness of the sputtered Ti layer (15 nm or 100 nm, leading to substrate named 15Ti@NM, 100Ti@NM, 15Ti@SSM and 100Ti@SSM) does not have any significant effect on the morphology of the CuO NPLs, as well as on subsequent electrodeposition of the Ru NPs.

Supplementary
These results shown in Supplementary Fig. 14,45 indicate that the CuO NPL growth is strongly affected by the chemistry (affinity) of the substrate surface, and surface treatment with sputtered Ti is required to obtain the catalysts characteristics of the cathodes previously developed using TM substrate. Supplementary Fig. 46. CuO NPLs growth on Ti-coated CM with Ti thickness of (a) 15 nm, and (b) 100 nm. The sputter coated Ti layer peeled off from CM surface, resulting inadequate for the subsequent electrodeposition of the Ru NPs.
CuO NPLs were not able to grow vertically on the surface of Ti-coated CM due to a peeling-off of sputtered Ti during chemical bath deposition. As expected from our previous cathode characterizations using the three-electrode configuration, the replacement of Ru@Cu-TM with its low-cost version Ru@Cu-30Ti@NM did not cause any relevant change of the AEL performance (e.g., 0.5 A/cm 2 at 1.69 V; 1 A/cm 2 at 1.85 V, Supplementary Fig.  48).